Kenneth H. Olsen

Rio Grande rift: An overview

Abstract The Rio Grande rift of the southwestern United States is one of the worlds principal continental rift systems. It extends as a series of asymmetrical grabens from central Colorado, through New Mexico, to Presidio, Texas, and Chihuahua, Mexico—a distance of more than 1000 km. Although the Rio Grande rift is closely related in timing and structural style to the contiguous Basin and Range extensional province, the two can be distinguished by a variety of geological and geophysical signatures. Rifts (both oceanic and continental) can be defined as elongate depressions overlying places where the entire lithosphere has ruptured in extension. The lithosphere of the Rio Grande rift conforms to this definition, in that: 1. (1) the crust is moderately thinned—Moho depths range from about 45 km under the flanks to about 33 km beneath the rift axis. 2. (2) anomalously low P n velocities (7.6–7.8 km s −1 ) beneath the rift and a long wavelength gravity low suggest that the asthenosphere is in contact with the base of the crust. The P-velocity is abnormally low (6.4–6.5 km s −1 ) in the lower half of the crust beneath the rift, suggesting high crustal temperatures. However, associated seismic and volcanologic data indicate the sub-rift lower crust is not dominated by a massive composite mafic intrusion such as is sometimes inferred for the East African rifts. Seismic and magnetotelluric data suggest the presence of a thin ( Structural development of the rift occurred mainly during two time intervals: the early phase beginning at −30 Ma. and lasting 10–12 m.y., and the late phase extending from −10 to 3 Ma. The early phase involved extensive low-angle normal faulting throughout the rift region which was subsequently offset by high-angle normal faulting during the later deformational event. Volcanism of the Rio Grande rift is minor compared to some other continental rifts. Most of the volcanism is basaltic and occurred less than about 5 m.y. ago. Compositions range from alkalic to tholeiitic, with no unique spatial or temporal pattern. Magmas were probably derived from a variety of depths, indicating an unintegrated heat source with only local melting. Basaltic andesites and related calc-alkaline rocks erupted in the southern rift between about 30 and 18 m.y. ago were not uniquely related to the rifting process. Rather, the thermal pulse which generated these magmas was part of the previous, subduction-related event. Our interpretation of existing data concerning the evolution of the Rio Grande rift does not fit either simple active or passive “end-member” models. In particular, there is no compelling evidence for a major thermal event in the mantle uniquely associated with rifting. Yet heat—inherited from the immediately-preceding deformational regime—was certainly a critical factor in, and was probably a necessary condition for, rifting.

A comparative study of the Rio Grande and Kenya rifts

Abstract Since they are two of the prominent continental rifts which are active today, the Kenya and Rio Grande rifts have been the subject of many recent studies. There are many gaps in our knowledge, but the data available make a comparative analysis worthwhile. Although they are part of much larger extensional regimes, these rifts are of similar dimensions and extent. In addition to these obvious similarities, geophysical data suggest the crustal and upper mantle structures of these features are also similar. They both are associated with relatively localized crustal thinning and much broader zones of lithospheric thinning. Seismic and gravity data indicate that the basement structure beneath both rift valleys is very complex, and recent studies have stressed the role of low-angle normal faulting in their structural development. However, geologic data and earthquake focal mechanisms indicate that high-angle normal faults and strike-slip faulting are also important. However, as one looks at the tectonic evolution and volcanic histories of the rifts, many differences are apparent. Volcanism has been virtually continuous in the Kenya rift area for over 20 Ma and the total volume of extrusives is over 200,000 km 3 . Unlike the Kenya rift, the Rio Grande rift formed in a region which was already both tectonically and magmatically active. Thus, there is uncertainty about what constitutes rift volcanism in the Rio Grande rift. However, the volume of extrusives generally accepted to be associated with this rift is about 5–10% that of the Kenya rift. The compositions of the volcanics are also very different in these rifts, and the Kenya rift displays migration of volcanic activity, and compositional variations in time which have not been recognized in the Rio Grande rift. The volcanic activity in the Rio Grande rift post-dated most of the faulting, whereas faulting and volcanism display complex interactions in Kenya. The nature and timing of uplift are important but difficult questions in both rifts.

The upper mantle structure of the central Rio Grande rift region from teleseismic P and S wave travel time delays and attenuation

The lithosphere beneath a continental rift should be significantly modified due to extension. To image the lithosphere beneath the Rio Grande rift (RGR), we analyzed teleseismic travel time delays of both P and S wave arrivals and solved for the attenuation of P and S waves for four seismic experiments spanning the Rio Grande rift. Two tomographic inversions of the P wave travel time data are given: an Aki-Christofferson-Husebye (ACH) block model inversion and a downward projection inversion. The tomographic inversions reveal a NE-SW to NNE-SSW trending feature at depths of 35 to 145 km with a velocity reduction of 7 to 8% relative to mantle velocities beneath the Great Plains. This region correlates with the transition zone between the Colorado Plateau and the Rio Grande rift and is bounded on the NW by the Jemez lineament, a N52°E trending zone of late Miocene to Holocene volcanism. S wave delays plotted against P wave delays are fit with a straight line giving a slope of 3.0 ± 0.4. This correlation and the absolute velocity reduction imply that temperatures in the lithosphere are close to the solidus, consistent with, but not requiring, the presence of partial melt in the mantle beneath the Rio Grande rift. The attenuation data could imply the presence of partial melt. We compare our results with other geophysical and geologic data. We propose that any north-south trending thermal (velocity) anomaly that may have existed in the upper mantle during earlier (Oligocene to late Miocene) phases of rifting and that may have correlated with the axis of the rift has diminished with time and has been overprinted with more recent structure. The anomalously low-velocity body presently underlying the transition zone between the core of the Colorado Plateau and the rift may reflect processes resulting from the modern (Pliocene to present) regional stress field (oriented WNW-ESE), possibly heralding future extension across the Jemez lineament and transition zone.

Modeling short-period crustal phases (P, Lg) for long-range refraction profiles

Abstract The short-period seismic phases known as P and Lg are often recorded at distances of 200–1000 km on long-range refraction profiles and are usually the largest-amplitude features on record sections for this distance range. P and Lg propagate as multiply reflected compressional and shear waves in a crustal waveguide whose principal boundaries are the Moho and the free surface. Equivalently, they can be interpreted as the interference pattern produced by a superposition of higher-mode P, SV and SH waves propagating in a leaky waveguide. For compressional waves, the waveguide efficiency is a strong function of frequency and depends on the presence or absence of low-velocity layers within a few kilometers of the surface, such as deep sedimentary sections commonly found in active tectonic areas. Such low-velocity surface layers create constructive interference effects for upcoming P waves incident at near grazing angles at the free surface and lead to efficient P propagation. Several good examples of strong P phases can be found on long-range refraction profiles for the tectonically active western United States; the 550 km profile eastward from SHOAL to Delta, UT is analyzed here. We have used a modified reflectivity-method computer program to model crustal phases for the SHOAL-Delta profile. The reflectivity technique accounts for all body and surface waves contributing to the short-period seismograms. It is found that the synthetic waveforms realistically model the observed P characteristics. In this case, the decay of P amplitudes with distance appears to be dominated by surface-reflection leakage from the waveguide rather than by anelastic attenuation due to Q of crustal rocks.

The role of seismic refraction data for studies of the origin and evolution of continental rifts

Abstract Universal attributes of continental rift systems are crustal extension with associated asthenospheric upwelling or modification of the mantle beneath the rift axis. Continental rifting is thus greatly controlled, if not driven, by geodynamic processes in the lower lithosphere. Seismic refraction profiling is a technique for obtaining high-resolution data on structures, seismic velocities, and petrologic conditions in the deep crust and upper mantle. Status of presently available seismic refraction data in four major Cenozoic continental rift systems (Rhinegraben of central Europe, Rio Grande rift of southwestern United States, Baikal rift of Eastern Siberia, and Kenya rift of East Africa) is reviewed in the light of extent of coverage in area and depth, reliability of velocity measurements, and outstanding structural and geodynamical problems that remain unanswered for each system. Evidence of crustal thinning, anomalous mantle at the Moho, as well as shallow and midcrustal velocity anomalies exist in all four examples. The Rhinegraben is the best constrained system from the point of view of seismic structure models. Seismic refraction structure data for the Kenya rift are sparse and raise more questions than answers. Since extensive geological, volcanological, and petrological data available for the East Africa system usually make it the standard for models of continental rifting, modern, high-resolution, deep seismic refraction profiles in Kenya are urgently needed in order to advance understanding of basic continental rifting processes.